Transflective liquid crystal display device

ABSTRACT

A liquid crystal display (LCD) device comprises a first substrate including a thin film transistor (TFT), a second substrate, and a liquid crystal (LC) layer of liquid crystal molecules. The LC layer is interposed by the first and second substrates. The first substrate includes a reflective electrode in a reflective region and a transmissive electrode in a transmissive region. The LC layer includes a first group of liquid crystal molecules aligned in the reflective region to provide a first retardation and a second group of liquid crystal molecules aligned in the transmissive region to provide a second retardation. The second retardation is different from the first retardation.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to liquid crystal display (LCD)devices and more particularly to transflective LCD devices, which usethe reflected and transmitted brightness.

[0003] 2. Description of the Related Art

[0004] Reflective LCD devices use the reflected brightness from roomlight, while transmissive LCD devices use the transmitted brightnessfrom an internal light source, such as a backlight.

[0005] The reflective LCD devices, which are used as display devices ofportable information terminals, are advantageous over the transmissiveLCD devices, in power consumption, thickness and weight. Such advantagesderive mainly from the elimination of a backlight. However, thetransmissive LCD devices are advantageous in visibility in darkenvironment.

[0006] Commonly, the LCD devices include a layer of liquid crystal (LC)molecules. Examples of modes are a twisted nematic (TN) mode, a singlepolarizing plate mode, a super twisted nematic (STN) mode, a guest host(GH) mode, a polymer dispersed liquid crystal (PDLC) mode, and acholesteric mode. A switching element is provided per pixel to drive theliquid crystal layer. A reflector or a backlight is provided within oroutside of the LC cell. To produce fine and high visibility in image,the LCD devices employ an active matrix drive system, in which switchingelements, such as, thin film transistors (TFTs) or metal/insulator/metal(MIM) diodes, are attached to each pixel to switch one “on” or “off”. Insuch LCD devices, a reflector or a backlight accompanies the activematrix drive system.

[0007] One example of known transflective LCD devices is illustrated inFIGS. 14 and 15. The same transflective LCD device is found in Kubo etal. (U.S. Pat. No. 6,195,140 B1 issued Feb. 27, 2001) and JP 2955277 B2.Kubo et al. illustrates the same structure in FIGS. 1 and 29 andprovides a description on FIG. 1 from line 49 of column 8 to line 11 ofcolumn 11 and a description on FIG. 29 in lines 53 to 63 of column 27.JP 2955277 B2 illustrates the same structure in FIGS. 1 and 10. Both JP2955277 B2 and Kubo et al. claim priority based on JP patent applicationNo, 9-201176 filed Jul. 28, 1997.

[0008]FIG. 14 is a plan view of one pixel portion of an active matrixsubstrate, illustrating gate lines 4 and source lines 5 that aredisposed along the peripheries of pixel electrodes 3 and cross eachother at right angles. TFTs 6 are formed in the vicinity of therespective crossings of the gate and source lines 4 and 5. A gateelectrode and a source electrode of each TFT 6 are connected to thecorresponding one gate line 4 and the corresponding one source line 5,respectively. Each of the pixel electrodes 3 includes a reflectiveregion 7 formed of a metal film and a transmissive region 8 formed ofindium/tin oxide (ITO).

[0009] In the reflective mode, the room light passes through the LClayer to the reflective regions 7 of the pixel electrodes 3. At thereflective regions 7, this light is reflected and returns through the LClayer to a viewer. In the transmissive mode, the light from a backlightpasses though the transmissive regions 8 of the pixel electrodes 3 andthe LC layer to the viewer.

[0010] In the reflective regions 7, the room light and the returnedlight pass through the LC layer in the opposite directions beforereaching the viewer. In the transmissive regions 8, the light from thebacklight passes through the LC layer once before reaching the viewer.When both reflective and transmissive modes are used simultaneously, thedifference in optical path makes it difficult to optimize the output,such as brightness and contrast. One approach to this problem is foundin the structure shown in FIG. 15. According to this structure, thethickness of the LC layer dr in the reflective regions 7 is differentfrom the thickness of the LC layer df in the transmissive region 8. Inthe reflective regions 7, the thickness dr is adjusted by adjusting thethickness of an insulating layer 17, which lies between a transmissiveelectrode 19 and on a reflective electrode 1. In FIG. 15, the referencenumeral 25 indicates a counter electrode.

[0011] To eliminate the difference in optical path, the setting is suchthat the ratio between the thickness dr and the thickness df is about1:2. This requires that the insulating layer 17 be thick almost as muchas half the thickness of the LC layer. Thus, in the reflective regions7, the insulating layer 17 with several micron meters thick is required,resulting in an increased fabrication processes. Besides, the provisionof such insulating layer 17 in each reflective region 7 prevents theoverlying transmissive electrode 19 from having surface flatness. Thesurface of the transmissive electrode 19 is coated with material to forman alignment film. The surface of the alignment film is not flat. Withsuch alignment film. robbing method may not provide a degree ofalignment of LC molecules as high as expected.

[0012] An object of the present invention is to provide a liquid crystaldevice in which the difference in optical path between the reflectivemode and transmissive mode is reduced with the plane surface of asubstrate kept.

SUMMARY OF THE INVENTION

[0013] According to one exemplary implementation of the presentinvention, a liquid crystal display (LCD) device comprises a firstsubstrate including a thin film transistor (TFT), a second substrate,and a liquid crystal (LC) layer of liquid crystal molecules. The LClayer is interposed by the first and second substrates. The firstsubstrate includes a reflective electrode in a reflective region and atransmissive electrode in a transmissive region. The LC layer includes afirst group of liquid crystal molecules aligned in the reflective regionto provide a first retardation and a second group of liquid crystalmolecules aligned in the transmissive region to provide a secondretardation. The second retardation is different from the firstretardation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of exemplary implementations of the invention as illustratedin the accompanying drawings. The drawings are not necessarily scale,emphasis instead being placed upon illustrating the principles of theinvention.

[0015]FIG. 1 is a sectional view of one pixel portion of a transflectiveLCD device, illustrating a first exemplary implementation of the presentinvention.

[0016] FIGS. 2(a) to 2(f) are views illustrating fabrication steps forthe first implementation.

[0017]FIG. 3 is a similar view to FIG. 1 illustrating differentalignment modes of LC molecules in the first implementation, one withineach of reflective regions and the other within each of transmissiveregions.

[0018]FIG. 4 is a sectional view of one pixel portion of a transflectiveLCD device, illustrating a second exemplary implementation of thepresent invention.

[0019] FIGS. 5(a) to 5(f) are views illustrating fabrication steps forthe second implementation.

[0020]FIG. 6 is a sectional view of one pixel portion of a transflectiveLCD device, illustrating a third exemplary implementation of the presentinvention.

[0021] FIGS. 7(a) to 7(f) are views illustrating fabrication steps forthe third implementation.

[0022]FIG. 8 is a very simplified view of one pixel portion of atransflective LCD device, illustrating a fourth exemplary implementationof the present invention.

[0023] FIGS. 9(a) to 9(i) are oversimplified views illustrating otherpossible arrangements of polarizers and quarter-wave plates.

[0024]FIG. 10 is a very simplified view of one pixel portion of atransflective LCD device, illustrating a fifth exemplary implementationof the present invention.

[0025]FIG. 11 is a graphical representation of varying of the strengthof output light I_(λ) versus different values of thickness of LC layer,and varying of the strength of output light I_(p) versus differentvalues of thickness of the LC layer.

[0026] FIGS. 12(a) and 12(b) are very simplified views of one pixelportion of a transflective LCD device, illustrating two embodiments of asixth exemplary implementation of the present invention.

[0027]FIG. 13 is a very simplified view of one pixel portion of atransflective LCD device, illustrating a seventh exemplaryimplementation of the present invention.

[0028]FIGS. 14 and 15 are views illustrating the discussed prior art.

DESCRIPTION OF THE EXEMPLARY IMPLEMENTATIONS

[0029] Referring to the accompanying drawings, the same referencenumerals are used to designate same or similar parts or portionsthroughout each view of FIGS. 1 to 13 for the sake of brevity ofdescription.

First Implementation of the Invention

[0030] With reference to FIGS. 1 to 3, the first implementation of atransflective LCD device of the invention is described.

[0031] As shown in FIG. 1, this LCD device includes, within the device,a lower side (or TFT) substrate 11, a counter substrate 12, and a LClayer 13 interposed between the substrates 11 and 12. The LCD deviceemploys an active matrix drive system, in which switching elements, suchas, TFTs or MIM diodes, are attached to each pixel.

[0032] Within one pixel portion illustrated in FIG. 1, the lower sidesubstrate 11 includes an insulating substrate 14, a protectiveinsulating film 15, a TFT 16, an insulating layer 17, a reflectiveelectrode 18, and a transmissive electrode 19, The substrate 14 iscoated with the protective film 15. Formed on the protective film 15 isthe TFT 16. The TFT 16 includes a gate electrode 16 a formed on thesubstrate 14, a drain electrode 16 b formed on the protective film 15, asemiconductor layer 16 c, and a source electrode 16 d. The protectivefilm 15 covers the gate electrode 16 a and provides insulation from thedrain electrode 16 b.

[0033] The insulating layer 17 covers the TFT 16. The insulating layer17 is formed with a contact hole 20 exposing a surface portion of theunderlying source electrode 16 d. The transmissive electrode 19 isformed over the insulating layer 17. A reflective electrode 18 is formedover a surface portion of the transmissive electrode 19. The reflectiveelectrode 18 is connected to the transmissive electrode 19, which inturn is connected via the contact hole 20 to the source electrode 16 d.The reflective electrode 18 and the transmissive electrode 19 are apixel electrode.

[0034] Material, such as polyimide, covers the reflective andtransmissive electrodes 18 and 19 to deposit an alignment film. Thealignment film is dividable into and consists of a vertical alignmentfilm portion 21 and a horizontal alignment film portion 22. Within areflective mode region, the vertical alignment film 21 coextends withthe reflective electrode 18. Within a transmissive mode region, thehorizontal alignment film 22 coextends with the exposed surface portionof the transmissive electrode 19. Another alignment film covers thesurface of the counter substrate 12, which faces the LC layer 13. Inthis implementation, the mode of alignment of LC molecules within thereflective mode region including the reflective electrode 18 is a HANalignment mode. The mode of alignment of LC molecules within thetransmissive mode region including the exposed transmissive electrode 19is a homogeneous alignment mode or a TN alignment mode.

[0035] Within a terminal region on the peripheral portions of the lowerside substrate 11, a gate terminal 23 is formed on the insulatingsubstrate 14, and a drain terminal 24 is formed on the protectiveinsulating film 15.

[0036] Viewing from the nearest side to the LC layer 13, the countersubstrate 12 includes a counter electrode 25, a color filter 26, and aninsulating substrate 27, which are formed one on another in this order.The incident light to the insulating substrate 27 of the countersubstrate 12 passes through the LC layer 13 to the reflective region ofeach pixel. At the reflective region, this light is reflected by thereflective electrode 18 and returns through the LC layer 13, counterelectrode 25 and counter substrate 12 to a viewer.

[0037] A backlight 28 is attached to the remote side of the undersidesubstrate 11 from the LC layer 13, The light from the backlight 28passes though the substrate 14, protective film 15, insulating layer 17and transmissive electrode 19 of the transmissive region of each pixelto the LC layer 13, and this light passes through the LC layer 13,transmissive electrode 25 and counter substrate 12 to the viewer.

[0038] With reference to the fabrication steps illustrated in FIGS. 2(a)to 2(f), the first implementation of the invention is further described.

[0039] Referring to FIG. 2(a), a TFT 16 is completed by forming a gateelectrode 16 a on an insulating substrate 14, coating the gate electrode16 a and insulating substrate 14 with a protective insulating film 15,and forming a drain electrode 16 b, a semiconductor layer 16 c, and asource electrode 16 d on the protective film 15.

[0040] Referring to FIG. 2(b), the TFT 16 is covered with an insulatinglayer 17. The insulating layer 17 is formed with a contact hole 20reaching the underlying source electrode 16 d. It is to be noted thatthe TFT 16 is one of various examples of a switching element, which maybe used in an active matrix drive system. Another example of theswitching element is a MIM diode.

[0041] Referring to FIG. 2(c), a transmissive electrode 19 of ITO isformed over the insulating layer 17. The material of the transmissiveelectrode 19 fills the contact hole 20 to establish electricalconnection between the transmissive electrode 19 and the sourceelectrode 16 d.

[0042] Referring to FIG. 2(d), with a portion being masked, a film of anelectrically conductive material such as aluminum (Al) is formed overthe unmasked portion of the transmissive electrode 19 to form areflective electrode 18. Another method is to form an electricallyconductive film over the whole surface area of the transmissiveelectrode 19. With a portion being masked, the electrically conductivematerial within the unmasked portion may be removed by etching.

[0043] Referring to FIG. 2(e), a polyimide material containinglong-alkyl side chain is coated onto the reflective electrode 18 andtransmissive electrode 19 and dried by heating to form a polyimidealignment film. The presence of long-alkyl side chain is known to inducea pretilt angle of LC almost as large as 90 degrees. Using a mask 29 tocover the alignment film overlying the reflective electrode 18, anultraviolet (UV) light is applied to the alignment film as indicated byarrows. Due to exposure to the UV light, separation of long-alkyl sidechain takes place within the unmasked portion overlying the transmissiveelectrode 19, resulting in a considerable reduction in pretilt angle inthe LC.

[0044] Referring to FIG. 2(f), after exposure to UV light, the mask 29is removed, and the polyimide alignment film is rubbed in a directionfor the LC molecules to align. That portion of the polyimide alignmentfilm that coats the reflective electrode 18 becomes a vertical alignmentfilm 21 because it was not exposed to the UV light. The remainingportion of the polyimide alignment film that coats the transmissiveelectrode 19 becomes a horizontal alignment film 22. In theimplementation, the rubbing treatment follows the exposure to UV light.The same result occurs by exposing the polyimide alignment film to UVlight after rubbing treatment. This is because the separation oflong-alkyl side chain takes place by the UV light exposure treatment inthe same manner before or after the rubbing treatment.

[0045] The polyimide material coated on the counter substrate 12 becomesa horizontal alignment film 22 due to the exposure to UV light and therubbing treatment. As a result, as shown in FIG. 3, the mode ofalignment of LC molecules within the reflective mode region includingthe reflective electrode 18 becomes a HAN alignment mode, and the modeof alignment of LC molecules within the transmissive mode regionincluding the exposed transmissive electrode 19 becomes a homogeneousalignment mode or a TN alignment mode.

[0046] Different orientations of alignment of the LC molecules providedifferent reflective indexes. The fabrication process described inconnection with FIGS. 2(a) to 2(f) has provided different modes ofalignment of LC molecules in the reflective mode region above thereflective electrode 18 and in the transmissive mode region above thetransmissive electrode 19. Using different indexes of refractionprovided by different modes of alignment of LC molecules, differentvalues in retardation (Δn×d) are provided in the reflective mode regionand in the transmissive mode region. Thus, with the same thickness ofthe LC cell, sufficiently high brightness is provided in the reflectivemode as well as in the transmissive mode.

[0047] In the present application, the expression that “a difference inmode of alignment of LC molecules” is intended to mean a difference intwist angle of the same alignment mode, for example, TN mode, whichcauses a difference in retardation. Such difference in twist angle ofthe same alignment mode can be obtained by rubbing the counter substratein one direction and subjecting the alignment film of the lower side orTFT substrate to optimal treatment. First step of the treatment is toexpose the alignment film on the TFT substrate to liner polarized lightsuch that the twist angle of LC molecules as much as about 70 degreestakes place. This step is followed by a second step of exposing thelaminated film, with its reflective region masked, to linear polarizedlight such that the twist angle of about zero degree takes place in theunmasked transmissive region. In this manner, the twist angle of LCmolecules becomes about 70 degrees in the reflective region, and thetwist angle of the LC molecules in the transmissive region becomes aboutzero degree. In this manner, a difference in retardation between thereflective region and the transmissive region is accomplished,

Second Implementation of the Invention

[0048] With reference to FIGS. 4 to 5(f), the second implementation of atransflective LCD device of the invention is described. The secondimplementation is substantially the same as the first implementationexcept the provision of a color filter 30 instead of the insulatinglayer 17.

[0049]FIG. 4 is the same view as FIG. 1, illustrating that a lower sidesubstrate 11 includes the color filter 30 instead of the insulatinglayer 17. The color filter 30 covers a TFT 16 and it is formed with acontact hole 20 exposing a surface portion of a source electrode 16 d.

[0050] With reference to the fabrication steps illustrated in FIGS. 5(a)to 5(f), the second implementation of the invention is furtherdescribed. The fabrication steps illustrated in FIGS. 5(a) to 5(f) aresubstantially the same as those illustrated in FIGS. 2(a) to 2(f),respectively.

[0051] Referring to FIG. 5(b), the TFT 16 is covered with a color filter30. The color filter 30 is formed with a contact hole 20 reaching theunderlying source electrode 16 d. The color filter 30 is fabricated bycolor-resist method. In the color-resist method, a photolithographytechnique is used to form the color patterns. The color-resist is madeby diffusing pigment (red or green or blue or black) in a photosensitiveacrylic polymer resin.

[0052] The second implementation is different in optical path from thefirst implementation. In the second implementation, in the transmissivemode, light from a backlight 28 passes through the color filter 30 ofthe under side or TFT substrate 11 before passing through a color filter26 of a counter substrate 12. In the reflective mode, the incident lightto the counter substrate 12 passes through the color filter 26, and thereflected return light passes again through the color filter 26. In eachof the modes, light passes through one color filter and then through thesame or another color filter, making color correction easy foreliminating a difference in color expression between the two modes orfor independent color correction in each of the modes.

Third Implementation of the Invention

[0053] With reference to FIGS. 6 to 7(e), the third implementation of atransflective LCD device of the invention is described. The thirdimplementation is similar to the second implementation in that a colorfilter 30 is used instead of the insulating layer 17 used in the firstimplementation. Comparing FIG. 6 to FIG. 4 clearly reveals that, in thethird implementation, a counter substrate 12 does not have a colorfilter, and a lower side or TFT substrate 11 has a reflective electrode18 formed on the surface of a protective insulating film 15. Thereflective electrode 18 is in contact with a source electrode 16 d of aTFT 16 and connected to a transmissive electrode 19 via a contact hole20 formed through the color filter 30. The color filter 30 underlyingthe transmissive electrode 19. As readily seen from FIG. 6, thethickness of the color filter 30 overlying the reflective electrode 18in the reflective region is less than the thickness of the color filter30 in the transmissive region. This difference in the thickness of colorfilter 30 can be adjusted by altering the thickness of the reflectiveelectrode 18.

[0054] The third implementation is different in optical path from thesecond implementation. In the third implementation, in the transmissivemode, light from a backlight 28 passes through the color filter 30 ofthe under side or TFT substrate 11 once. In the reflective mode, theincident light to the counter substrate 12 passes through the colorfilter 30, and the reflected return light passes again through the colorfilter 30. As mentioned above, altering the thickness of the reflectiveelectrode 18 can provide a desired difference in the thickness of colorfilter 30, making color correction easy for eliminating a difference incolor expression between the two modes. Beside, there is no need toprovide a color filter in the counter substrate 12, leading to areduction in fabrication cost. An error inherent with positioning acolor filter within the counter substrate no longer exists, providingenhanced display.

[0055] With reference to the fabrication steps illustrated in FIGS. 7(a)to 7(e), the third implementation of the invention is further described.FIGS. 7(a) to 7(c) are substantially the same as FIGS. 5(a) to 5(c),respectively, except the formation of the reflective electrode 18 inFIG. 7(a) in the third implementation. In the second implementation, thereflective electrode 18 is formed on the transmissive electrode 19 inFIG. 5(d). Accordingly, this implementation does not have a fabricationstep corresponding to the fabrication step illustrated in FIG. 5(d).FIGS. 7(d) and 7(e) are substantially the same as FIGS. 5(e) and 5(f),respectively.

[0056] Referring to FIG. 7(a), a gate electrode 16 a is formed on aninsulating substrate 14. The gate electrode 16 a and the insulatingsubstrate 14 are coated with a protective insulating film 15. Areflective electrode 18 is formed on the on the protective film 15 incontact with a source electrode 16 d. Forming a drain electrode 16 b anda semiconductor layer 16 c completes a TFT 16.

[0057] Referring to FIG. 7(b), the TFT 16 and reflective electrode 18are covered with a color filter 30. The color filter 30 is formed with acontact hole 20 reaching the underlying reflective electrode 18.

[0058] Referring to FIG. 7(c), a transmissive electrode 19 of ITO isformed over the color filter 30. The material of the transmissiveelectrode 19 fills the contact hole 20 to establish electricalconnection between the transmissive electrode 19 and the reflectiveelectrode 18.

[0059] The fabrication steps of FIGS. 7(d) and 7(e) are substantiallythe same as the fabrication steps of FIGS. 5(e) and 5(f), which in turnare substantially the same as the fabrication steps of FIGS. 2(e) and2(f). Accordingly, a description on FIGS. 7(d) and 7(e) is herebyomitted for brevity,

Fourth Implementation of the Invention

[0060] With reference to FIG. 8, the fourth implementation of atransflective LCD device of the invention is described. Thisimplementation is applicable to any one of the first, second and thirdimplementations if the mode of a LC layer 13 is of the TN type.According to the fourth implementation, a lower side substrate 11includes a polarizer 31 and a quarter-wave plate 32, and a countersubstrate 12 includes another polarizer 31 and another quarter-waveplate 32.

[0061] In the lower side substrate 11 shown in FIG. 8, the polarizer 31is positioned between an insulating substrate 14 and a backlight 28. Thequarter-wave plate 32 is positioned between the insulating substrate 14and an insulating layer 17 (or a color filter 30, see FIGS. 4 and 6). Inthe counter substrate 12 shown in FIG. 8, the quarter-wave plate 32 ispositioned on the nearest side of an insulating substrate 27 to the LClayer 13. The polarizer 31 is positioned on the remote side of theinsulating substrate 24 from the LC layer 13.

[0062] The polarizers 31 are oriented orthogonally to each other. Eachof the polarizers 31 has a transmission axis. In the lower sidesubstrate 11, the fast and slow axes of the quarter-wave plate 32 shouldeach be oriented at an angle of substantially 45 degrees relative to theorientation of the transmission axis of the polarizer 31. In the countersubstrate 12, the fast and slow axes of the quarter-wave plate 32 shouldeach be oriented at an angle of substantially 45 degrees relative to theorientation of the transmission axis of the polarizer 31.

[0063] In the reflective mode, when the LC layer 13 is twisted, thepolarizer 31 of the counter substrate 12 receives ambient light andlinearly polarizes the light. The quarter-wave plate 32 converts thelinearly polarized light into right handed circularly polarized light.The twisted LC layer 13 converts the right handed circularly polarizedlight into linearly polarized light. At the reflective electrode 18, thelinearly polarized light is reflected and returns to the twisted LClayer 13. The twisted LC layer 13 converts the linearly polarized returnlight to right handed circularly polarized return light. Thequarter-wave plate 32 of the counter substrate 12 converts the righthanded circularly polarized return light to linearly polarized returnlight. The linearly polarized return light passes through the polarizer31 of the counter substrate 12 to a viewer.

[0064] In the reflective mode, when the LC layer 13 is verticallyaligned, the polarizer 31 of the counter substrate 12 receives ambientlight and linearly polarizes the light. The quarter-wave plate 32converts the linearly polarized light into right handed circularlypolarized light. The right handed circularly polarized light passesthrough the vertically aligned LC layer 13, is reflected at thereflective electrode 18 to become left handed circularly polarizedreturn light. The left handed circularly polarized return light passesthrough the vertically aligned LC layer 13. The quarter-wave plate 32 ofthe counter substrate 12 converts the left handed circularly polarizedreturn light to linearly polarized return light. This linearly polarizedreturn light cannot pass through the polarizer 31 of the countersubstrate 12.

[0065] In the transmissive mode, when the LC layer 13 is twisted, thepolarizer 31 of the lower side substrate 11 receives light from thebacklight 28 and linearly polarizes the light. The quarter-wave plate 32converts the linearly polarized light into left handed circularlypolarized light. The twisted LC layer 13 converts the left handedcircularly polarized light into right handed circularly polarized light.The quarter-wave plate 32 of the counter substrate 12 converts the righthanded circularly polarized light to linearly polarized light. Thelinearly polarized light passes through the polarizer 31 of the countersubstrate 12 to the viewer.

[0066] In the transmissive mode, when the LC layer 13 is verticallyaligned, the polarizer 31 of the lower side substrate 11 receives lightfrom the backlight 28 and linearly polarizes the light. The quarter-waveplate 32 converts the linearly polarized light into left handedcircularly polarized light. The left handed circularly polarized lightpasses through the vertically aligned LC layer 13. The quarter-waveplate 32 of the counter substrate 12 converts the left handed circularlypolarized light to linearly polarized light. This linearly polarizedlight cannot pass through the polarizer 31 of the counter substrate 12.

[0067] As shown in FIG. 8, the fourth implementation has insidequarter-wave plates 32, which are protected against UV radiation andhumidity by the insulating substrates 14 and 27 and the polarizers 31.

[0068] In the fourth implementation, the quarter-wave plates 32 areseparated from the respective polarizers 31 and positioned on the nearsides of the insulating substrates 14 and 27 to the LC layer 13. Thisarrangement no longer requires adhesive that was used between thepolarizer and the quarter-wave plate, providing greater freedom inselecting an appropriate material of the quarter-wave plate.

[0069] The quarter-wave plate is formed of material exhibiting liquidcrystal property, which will induce alignment of LCs. Thus, if thequarter-wave plates are positioned in place of alignment films, theprovision of alignment films and rubbing treatment may be eliminated. Inthe case LC cells are in the form of 90 degrees twist structure, it isno longer needed to subject the underside and counter substrates toalignment treatment. In the case LC cells are of the HAN type, therubbing treatment may be eliminated.

[0070] It is possible to arrange quarter-wave plates in a manner tointerpose LC cells. This arrangement is effective in preventing lightfrom the adjacent pixels to pass through the relatively thin insulatingsubstrates, each having 500 to 700 μm thick.

[0071] FIGS. 9(a) to 9(i) are oversimplified views illustrating otherpossible arrangements of polarizers 31 and quarter-wave plates 32.

Fifth Implementation of the Invention

[0072] With reference to FIG. 10, the fifth implementation of atransflective LCD device of the invention is described. Thisimplementation is applicable to any one of the first, second and thirdimplementations if the mode of a LC layer 13 is of the TN type.According to the fifth implementation, a lower side substrate 11includes a polarizer 31 and a quarter-wave plate 32, and a countersubstrate 12 includes another polarizer 31 and another quarter-waveplate 32. However, the quarter-wave plate 32 of the counter substrate 12is not provided in the transmissive region.

[0073] In the lower side substrate 11 shown in FIG. 8, the polarizer 31is positioned between an insulating substrate 14 and a backlight 28. Thequarter-wave plate 32 is positioned between the insulating substrate 14and an insulating layer 17 (or a color filter 30, see FIGS. 4 and 6). Inthe counter substrate 12 shown in FIG. 8, the quarter-wave plate 32 ispositioned on the nearest side of an insulating substrate 27 to the LClayer 13. The polarizer 31 is positioned on the remote side of theinsulating substrate 24 from the LC layer 13.

[0074] A photoresist and etching technique is used to remove the portionof the quarter-wave plate 32 of the counter substrate 12 to form anopening in the transmissive region.

[0075] The following two paragraphs provide a description on therelationship between the strength of output light and quarter-wave platein the transmissive mode.

[0076] In the transmissive mode, light from a backlight 28 passesthrough the polarizer 31 and quarter-wave plate 32 of the lower sidesubstrate 11 to a LC layer 13. After passing through the LC layer 13,light passes through the quarter-wave plate 32 and the polarizer 31 ofthe counter electrode 12. The strength of output light I_(λ) isexpressed as,

I _(λ)=(1/2)[(Γ/2)×(1/X)×sin X] ²  Eq. 1

[0077] where,

[0078] λ=Wavelength of light

[0079] (Δn×d)=Retardation of LC layer 13

[0080] φ=Twist angle of LC molecules

[0081] Γ=2π(Δn×d)/λ

[0082] X=[φ²+(Γ/2)²]^(1/2)

[0083] In the transmissive mode, light from the backlight passes throughthe polarizer 31 of the underside substrate 11 to the LC layer 13, andafter passing through the LC layer 13, light passes through thepolarizer 31 of the counter substrate 12. In this case, the light doesnot pass any one of the quarter-wave plates 32. The strength of outputlight I_(p) is expressed as,

I _(p)=(1/2)[(1/X)×sin X] ²[φ²×cos²φ+sin²φ×(Γ/2)²]+sin²φ cos² X−φ sin2φcos X[(1/X)×sin X]  Eq. 2

[0084]FIG. 11 provides a curve interconnecting the plotted values ofI_(λ) calculated for different values of thickness of LC layer, andanother curve interconnecting the plotted values of I_(p) calculated fordifferent values of thickness of the LC layer. In the reflective mode,the thickness dr of the LC layer when the strength of output light isthe maximum is about 3 μm. When a LCD device is designed with thethickness of LC layer around 3 μm, the strength of output light I_(p)exceeds the strength of output light I_(λ).

[0085] Apparently, the above consideration suggests that, in the fifthimplementation of FIG. 10, removing the quarter-wave plate 32 of thelower side substrate 11 will increase the strength of output light intransmissive mode as well as in the reflective mode.

Sixth Implementation of the Invention

[0086] With reference to FIGS. 12(a) and 12(b), the sixth implementationof a transflective LCD device of the invention is described. Thisimplementation features the provision of three layer filter/reflector 33of cholesteric liquid crystal (CLC) layers which reflect left or righthanded circularly polarized blue, green and red light.

[0087] The three-layer filter/reflector 33 is positioned between aninsulating substrate 14 of a lower side substrate 11 and a backlight 28.In a counter substrate 12, a quarter-wave plate 32 is positioned on thenear side of an insulating substrate 27 to a LC layer 13.

[0088] In the sixth implementation, the three-layer filter/reflector 33of CLC layers have replaced a quarter-wave plate and a polarizer in thelower side substrate 11. In the transmissive mode, the reflectedcircularly polarized light passes to the viewer. To prevent this, twoquarter-wave plates 32 are positioned on the near side of thethree-layer filter/reflector 33 of CLC layers to the LC layer 13 asillustrated in FIG. 12(a). As illustrated in FIG. 12(b), a singlequarter-wave plate 32 may be positioned on the near side of thethree-layer filter/reflector 33 of CLC layers to the LC layer 13. Inthis case, a quarter-wave plate may be removed from counter substrate12.

Seventh Implementation of the Invention

[0089] With reference to FIG. 13, the seventh implementation of atransflective LCD device of the invention is described. Thisimplementation features the provision of two quarter-wave plates 32 andthree-layer filter/reflector 33 of CLC layers, and the elimination of aquarter-wave 32 of a counter substrate 12 in the transmissive region.

[0090] The three-layer filter/reflector 33 is positioned between aninsulating substrate 14 of a lower side substrate 11 and a backlight 28.The quarter-wave plate 32 is positioned between an insulating substrate14 and the three-layer filter/reflector 33. In a counter substrate 12, aquarter-wave plate 32 is positioned on the near side of an insulatingsubstrate 27 to a LC layer 13.

[0091] A photoresist and etching technique is used to remove the portionof the quarter-wave plate 32 of the counter substrate 12 to form anopening in the transmissive region.

[0092] While the present invention has been particularly described, inconjunction with exemplary implementations, it is evident that manyalternatives, modifications and variations will be apparent to thoseskilled in the art in light of the foregoing description. It istherefore contemplated that the appended claims will embrace any suchalternatives, modifications and variations as falling within the truescope and spirit of the present invention.

[0093] This application claims the priority of Japanese PatentApplication No. P2001-251089, filed Aug. 22, 2001, the disclosure ofwhich is hereby incorporated by reference in its entirety.

What is claimed is:
 1. A liquid crystal display (LCD) device,comprising: a first substrate including a thin film transistor (TFT); asecond substrate; and a liquid crystal (LC) layer of liquid crystalmolecules, the LC layer being interposed by the first and secondsubstrates; the first substrate including a reflective electrode in areflective region and a transmissive electrode in a transmissive region,the LC layer including a first group of liquid crystal molecules alignedin the reflective region to provide a first retardation and a secondgroup of liquid crystal molecules aligned in the transmissive region toprovide a second retardation that is different from the firstretardation.
 2. The LCD device as claimed in claim 1, wherein the firstsubstrate further includes an insulating layer formed over the TFT, andthe reflective electrode is formed on the insulating layer, and whereinthe second substrate includes a color filter.
 3. The LCD device asclaimed in claim 1, wherein the first substrate further includes a colorfilter formed over the reflective electrode and the TFT, the colorfilter having a first thickness in the reflective region and a secondthickness in the transmissive region, the first thickness being lessthan the first thickness.
 4. The LCD device as claimed in claim 1,wherein the first substrate further includes a color filter formed overthe TFT, and the reflective electrode is formed on the color filter, andwherein the second substrate includes another color filter.
 5. The LCDas claimed in claim 1, wherein said transmissive electrode iselectrically connected to the reflective electrode.
 6. The LCD device asclaimed in claim 1, wherein the liquid crystal molecules are aligned inat least one mode selected from homogeneous alignment mode, homeotropicalignment mode, TN alignment mode, HAN alignment mode, and OCB alignmentmode.
 7. The LCD device as claimed in claim 1, wherein the first groupof liquid crystal molecules is aligned in HAN alignment mode, and thesecond group of liquid crystal molecules is aligned in one ofhomogeneous alignment mode and TN alignment mode.
 8. The LCD device asclaimed in claim 1, wherein the second substrate includes a quarter-waveplate.
 9. The LCD device as claimed in claim 8, wherein the quarter-waveplate has an opening in the transmissive region.
 10. The LCD device asclaimed in claim B, further comprising cholesteric liquid crystal (CLC)layer on the remote side of the second substrate from the LC layer. 11.The LCD device as claimed in claim 8, wherein the first substrateincludes a second quarter-wave plate.
 12. The LCD device as claimed inclaim 9, further comprising cholesteric liquid crystal (CLC) layer onthe remote side of the second substrate from the LC layer, and a secondquarter-wave plate between the CLC layer and the first substrate.